1. Field of the Invention
This invention generally relates to dispensing apparatus and more specifically to dispensing apparatus capable of metering predetermined amounts of a material with accuracy.
2. Description of Related Art
Conventional positive displacement pumps available today for high accuracy dispensing applications are either piston-type or rotary screw-type pumps. Both types find uses in specific applications. For example, U.S. Pat. No. 5,499,745 (1996) to Derian et al. discloses an apparatus for mixing and dispensing two chemically reactive materials. This apparatus includes a metering unit for dispensing two-part polymeric materials from a dispensing module attached to the end of the robot arm. The metering unit includes a piston located concentrically about a shaft of a dual actuating piston assembly to provide the two polymeric materials in a fixed ratio. A variable rate dispenser compensates for changes in robot velocity or viscosity changes.
U.S. Pat. No. 5,819,983 (1998) to White et al. discloses a liquid dispensing system with a sealing augering screw. The augering screw serves both a metering function and a valving function and is axially movable between a position in which fluid can flow and a sealing position in which liquid flow through the nozzle is substantially prevented. The screw has a curved contour between its threads and a plurality of thread-defining channels to allow liquid to fill more completely around the screw thereby to dispense more liquid with fewer screw turns than prior designs.
U.S. Pat. No. 5,927,560 (1999) to Lewis et al. discloses a dispensing pump that dispenses a two-part epoxy and includes a pumping chamber in communication with a nozzle by means of a three-way valve. The pumping chamber includes an outlet, an internal volume, a pair of spaced directional seals located away from the outlet, an open volume residing between the seals and an external port opening the open volume to atmosphere. A stepped plunger extends axially within the pumping chamber. A first portion is sized to be received and aligned through a first directional seal so the plunger can move the first portion toward the outlet to close off the internal volume at the second seal. The second plunger portion is relatively smaller and transfers cross-sectional dimension than the inside diameter of the second seal to cooperate with the port to promote fluid removal from the internal volume during initial filling and priming of the pumping chamber. Once filled and primed, the plunger causes positive volume displacement of liquid from the pumping chamber and out of the nozzle, with very good repeatability and high fluid flow rates, independent of material viscosity, and/or temperature or viscosity changes. For refilling, the plunger is retracted by a volume which corresponds to the volume of material refilled into the internal volume.
U.S. Pat. No. 5,957,343 (1999) to Cavallaro discloses a controllable liquid dispensing device with a selectively adjustable material control device and a sealing device with a dispensing device. The sealing device is in communication with a switching device to move the sealing device between a first position and a second position. The first position of the sealing device allows the material control device to draw a volume of material into the dispensing device. The second position of the sealing device allows the material control device to force the volume of material to dispense from the dispensing device.
The foregoing patents disclose a number of possible approaches to various dispensing devices. However, they are not the best choices for many high accuracy and high throughput applications. Many such applications use high viscosity abrasive fluids that further exacerbate the problem because the materials tend to wear and increase the force or torque needed to drive the system to dispense material. Screw-type pumps lack the accuracy or the throughput needed for today's production processes. Although piston pumps offer better volumetric accuracy, they lack any feedback capability and are also slow. Piston pumps further generally have a large size, are difficult to use and impose real restrictions on the end user.